In a nice example of lateral thinking, Power Integrations has found a way to halve the space needed for electrolytic reservoir capacitors in universal mains ac-dc PSUs up to 65W.

PowerInt-MinEcap-circuit
It relies on automatically switching a low-voltage capacitor in parallel with a high voltage capacitor, springing from the observation that, for the same amount of stored energy between mains half-cycles:
a PSU running from 115V needs a larger capacitance that a 240V supply
a PSU running from 240V need a higher voltage capacitor than a 115V supply
The result is, said PowerInt, that a PSU that can run from both voltages needs a single capacitor with ~2x to 3x the physical volume of a capacitor in a similar supply designed to run from only 115V or only 240V.

In a 65W PSU, for example, it said that a 100μF 400V capacitor is needed if it must run from both high and low lines, while a single 22μF 400V can support 240V (176 – 264Vac) operation.

PowerInt-MinEcap-exampleIf a 100μF 160V (actually a pair of 47μF 160V in parallel, photo left) is automatically switched in parallel to support 115V (90 – 132Vac) operation, then the total physical volume of capacitors is half that of the 100μF 400V part.
There is no general formula for volume improvement, said the company, because each capacitor maker uses a limited range of can sizes, resulting in a steppy capacitance to volume and voltage to volume ratios.
To perform the automatic switching, PowerInt has created an integrated circuit called MIN1072M (branded ‘MinE-CAP), that includes a GaN HEMT power transistor.
“To get a low-enough Rds(on), we needed to use GaN,” Power Integrations marketing director Chris Lee told Electronics Weekly. “You can use GaN in ways that you just can’t use silicon.”
One of the ways the GaN transistor is used, is to modulate in-rush current to the 160V capacitor, removing the need for an up-front NTC thermistor in PSUs up to 65W, said Lee, who pointed out that a completely different front-end strategy is needed above 75W, when power factor correction front ends are often mandatory.
MIN1072M is 11.3 x 5.6mm and 2mm tall, and designed specifically to work with the company’s InnoSwitch 3 ac-dc controller chips (circuit top), but can be used with other controllers with extra circuitry.
PowerInt-MinEcap-cap-value-rangeWhen using the chip, the game becomes selecting the smallest bulk capacitor required for all possible high AC line voltages, then picking the amount of additional low-voltage energy storage capacitance that will be switched and protected by the MinE-CAP (graph right).
As an aside, the chip was originally envisaged switching once – either on or off at circuit power-up – and then stay in that state. But, according to Lee, applications have been found where it is advantageous for the chip to switch along with the 100Hz or 120Hz rectified mains.
When the mains is is prone to surges
In certain countries, India for example according to Power Integrations, mains voltage can peak well above 264Vac making 800V capacitors necessary at the front end.
For these markets, the company has a design that uses 2 x 47μF 160V in parallel, and then four 22μF 400V in series-parallel to produce a 22μF 800V capacitor for high voltage situations. The un-switched alternative is 20 x 22μF 400V capacitors forming a 110μF 800V bank, for example.
PowerInt MinEcap DER-626 ref designDER-626 – for normal mains voltages – read below
“MinE-CAP ICs are excellent for all locations with wide ranging input voltages,” said Power Integrations India director Bhaskar Thiagaragan. “In India we often design for voltages from 90Vac to 350Vac, with a generous surge de-rating above that. Engineers here often complain about the forest of high-voltage capacitors required. MinE-CAP dramatically reduces the number of storage components, and shields lower voltage capacitors from the wild mains voltage swings.”
PowerInt-MinEcap-DER626Back with less exciting mains voltages, the company has produced two detailed reference designs for universal ac-dc PSUs:
DER-626 is a credit card sized 65W USB PD/PSS adaptor 82 x 51 x 12mm (50cm3) (graph right, photo above)
DER-822 is a smaller 60W USB PD/PSS adaptor 51 x 26 x 22mm (30cm3)
“USB PD has driven a major market push towards small 65W chargers, and many companies have concentrated on increasing switching frequency to reduce the size of the flyback transformer,” said Lee. “MinE-CAP provides more volume saving than doubling the switching frequency, while actually increasing system efficiency.”